The manufacture of metal components, including soft magnetic metal components, using a metal powder as the raw material, i.e., powder metallurgy, has been used for decades. Powder metallurgy is an excellent method of shaping metals into a predetermined design because of an efficient use of energy and materials. Powder metallurgy provides metal components of near net shape, and, therefore, is a common method of manufacturing large volumes of close tolerance metal components.
The manufacture of a metal component by powder metallurgy includes three basic steps to convert a metal powder into a metal component. Each step is controlled such that the finished metal component conforms to design specifications both within a single production batch and also between production batches.
The first step is preparation of a metal powder mixture. The metal powder mixture typically includes: (1) the metal powder being used as the material of construction, and (2) a lubricant. The metal powder can be a single metal species or can be a combination of different types of metal species. The metal powder particles typically are spherical or near spherical in shape. As used hereafter, the term "spherical" designates both a spherical and a near spherical shape. The lubricant typically is added to minimize friction between the metal powder and the tooling during a compaction, or pressing, step. The lubricant is present in an amount of up to about 2% by weight of the metal powder mixture. Alternatively, a lubricant is omitted from the metal powder mixture, but is applied to the die wall and tooling.
After forming the metal powder mixture, the mixture is pressed in a die of predetermined shape. During the pressing operation, the spherical metal powder particles deform to form a compressed article, termed a "green compact," having about 40% the original height of the metal powder height. The shape of the green compact is determined by the geometry of the die. The green compact can be handled, but is fragile.
The density of the green compact (i.e., "green density") is determined primarily by the applied pressing load and the amount of lubricant. The ability of the green compact to maintain its predetermined shape without cracking, fracturing, or crumbling during handling is referred to as the "green strength" of the compact. If green strength is too low, the green compact easily crumbles or cracks when removed from the die, which makes manufacture into a metal component difficult to impossible.
After pressing, the green compact is subjected to an elevated temperature to form a metal component. The green compact is heated at a sufficiently high temperature and for a sufficient time to decompose, or pyrolyze, the lubricant, and to increase the density and strength of the metal component.
Conventionally, the green compact is heated in steps, initially to a first temperature to pyrolyze the lubricant, then to a second higher temperature to increase the density and strengthen the metal component, i.e., to sinter the metal component. A typical sintering furnace comprises a continuously running mesh belt which carries the green compacts through the furnace. Heating cycle times typically are about 1 to 3 hours, with about 20 to about 60 minutes at a sintering temperature in excess of 1000.degree. C. The sintered metal component, after cooling, then is subjected to optional secondary operations, such as deburring, to provide the final finished metal component.
The strength of a metal component is directly related to the density of the metal component, which in turn is directly related to the density of the green compact. Metal components manufactured by the above-described traditional powder metallurgy process, and using metal powder particles having spherical or near-spherical geometry, have a theoretical density of about 88% to about 92%. As used here and hereafter, the term "100% theoretical density" is defined as the density of the metal, metals, alloy, and/or alloys forming the metal component. "Percent (%) theoretical density" is defined as the ratio of green compact density, or metal component density, to the density of the metal, metals, alloy, and/or alloys from which the green compact or metal component is manufactured, multiplied by 100.
Metal components having a % theoretical density of about 88% to about 92% often exhibit low strength, and are susceptible to corrosion due to the porosity of the metal component. Such metal components are unsuitable for many practical applications because they are subject to failure. Persons skilled in the art have used various techniques, including "warm" pressing, pressing the metal component a second time (i.e., "sizing" or "restriking"), or hot isostatic pressing to increase the density of the metal component. However, each of the above-described techniques is more costly than a traditional powder metallurgy process, and provides metal components having a density typically no greater than about 96% of theoretical density.
Conventionally, metal powder particles used in powder metallurgy are spherical or near-spherical in shape. Spherical metal powder particles, typically minus 100 mesh, or about 200 microns or smaller, in size, are blended with a die lubricant and compacted into a predetermined shape. The amount of lubricant used with spherical metal powder is about 0.25% to about 2%, by weight of the metal powder mixture. After pressing, a green compact containing the spherical metal powder has a green density typically less than 92% of theoretical density. This relatively low green density is attributed to: (1) the resistance of spherical metal powder to efficiently compress to high densities in a die (i.e., spheres inherently resist compaction and arrays of spheres have substantial void spaces between the spheres), and (2) the relatively higher volume occupied by the low density lubricant (which decreases the overall density of the green compact). During heating and sintering, the lubricant is pyrolyzed, the metal particles coalesce or sinter together causing a slight volume decrease, and the density of the resulting metal component is increased, but typically to less than 93% of the theoretical density of the metal or metal alloy. The low density of the metal component adversely affects performance, and promotes corrosion due to a relatively high porosity.
Spherical metal powder particles having a size typically used in powder metallurgy (e.g., minus 100 mesh, or about 200 .mu.m, in size, or smaller) require relatively large amounts of lubricant because each powder particle must be coated with a minimum amount of lubricant, and a volume of spherical powder particles of this size has a large surface area-to-weight ratio. It is desirable to minimize the amount of lubricant in the metal powder mixture in order to minimize the die volume occupied by the lubricant, and thereby increase the density of the green compact. Investigators attempting to minimize the amount of lubricant present in the metal powder mixture have addressed the morphology, i.e., the size and shape, of the metal powder particles.
Krause et al. U.S. Pat. No. 5,594,186, incorporated herein by reference, discloses substantially linear, acicular metal particles having a substantially triangular cross section. The acicular particles can be used in the manufacture of metal components by powder metallurgy, and overcome the disadvantages of spherical metal particles. In particular, the acicular particles disclosed in Krause et al. U.S. Pat. No. 5,594,186 provide powder metallurgy components having a theoretical density at least 95% of theoretical density.
A volume of the acicular metal particles disclosed in Krause et al. U.S. Pat. No. 5,594,186 has a reduced surface area-to-weight ratio compared to a volume of spherical powder particles typically used in powder metallurgy, and, therefore, the amount of lubricant needed to coat the volume of acicular metal particles is reduced. The reduced amount of lubricant results in an increased green density, and, subsequently, an increased density of the finished metal component.
Metal components containing a magnetizable metal, prepared by powder metallurgy, and having a high theoretical density exhibit excellent magnetic properties. Investigators have found that magnetic properties are directly related to percent theoretical density, and, therefore, magnetic metal components prepared by a powder metallurgy process from the acicular metal particles disclosed in Krause et al. U.S. Pat. No. 5,594,186 are expected to exhibit excellent magnetic properties. But, metal components prepared from iron particles disclosed in Krause et al. U.S. Pat. No. 5,594,186 generally do not exhibit sufficient corrosion resistance to be useful in many commercial applications.
In particular, a powder metallurgy magnetic component exhibits good magnetic properties by exhibiting an induction at an applied field of 100 Oe (Oersteds), or B.sub.100, of at least about 12 kG (kilo-Gauss), and typically about 12 to about 15 kG; and a coercive force, or H.sub.c, measured from an applied field of 100 Oe of about 3 Oe or less, and typically about 1.5 to about 2.5 Oe. The higher the B.sub.100, and the lower the H.sub.c, the better the magnetic properties of the metal component.
In addition to good magnetic properties, the metal component also must demonstrate good corrosion resistance for practical applications. This is especially important in magnetic metal components such as rotor cores used in brushless dc motors, and sensor rings used in automotive antilock breaking systems (ABS)
Currently, methods are available to impart corrosion resistance to powder metallurgy metal components, but each method has a disadvantage. For example, a corrosion-resistant coating can be applied to the magnetic metal component. One example of a protective coating is a reactive coating, such as a phosphate or a steamed blue oxide coating. Another example is to simply apply a coating composition, such as a paint or an epoxy. The disadvantage of protective coatings is that if the coating is damaged, then corrosion of the metal component begins at the damaged area and spreads under undamaged areas of the coating.
Corrosion resistance can be imparted by steam treating the magnetic metal component in an oxygen-rich atmosphere. Steam treating oxidizes the component surfaces, including open pore surfaces that extend to the interior of the component. However, steam treating significantly decreases the magnetic properties of the metal component. See L. I. Frayman et al., "The Role of Secondary Operations in the Manufacturing of P/M Automotive Components for Soft Magnetic Applications," Presented at 1996 World Congress on Powder Metallurgy & Particulate Materials, Jun. 16-21, 1996, Washington, D.C.
Another common approach to impart corrosion resistance to a powder metallurgy component is to use a base powder composition that is itself corrosion resistant. For example, typical 400 series stainless steels exhibit excellent corrosion resistance, and metal components, such as ABS sensor rings, have been made of stainless steel. The serious disadvantages of this approach is the relatively poor magnetic properties of stainless steel compared to iron, and the relatively high cost of the stainless steel alloy, as well as increased manufacturing costs.
A third approach to impart corrosion resistance is infiltration of a magnetizable metal component with a corrosion-resistant metal or alloy. For example, infiltrating copper into an iron-based powder metallurgy metal component has been practiced for decades in order to increase the density, strength, corrosion resistance, machinability, and conductivity of the power metallurgy metal component.
In an infiltration process, the term "skeleton" refers to a compacted matrix of metal particles having a network of solid particles that remain throughout the infiltration process. The skeleton provides a system of interconnected pores and channels of a size range that permits unimpeded flow of a liquid metal by capillary action. The term "infiltrant" refers to a metal or alloy that infiltrates the pores of the skeleton. The infiltrant metal or alloy has a melting point below the melting point of the metal or alloy of the skeleton.
During infiltration, a mass of a liquid metal, i.e., an "infiltrant," flows through and fills the pore system of a solid-phase powder metallurgy metal compact, i.e., a "skeleton." Infiltration, therefore, is an excellent method of producing powder metallurgy metal components of near 100% theoretical density by filling the pores of a compressed metal skeleton with a lower-melting metal infiltrant. This technique has the advantage of increasing the density of the metal component without the expenditure of external work, like extra pressing operations, to reduce or close the void spaces present in pressed and sintered metal components.
Infiltration can be performed on either green or sintered metal compacts formed by powder metallurgy techniques. The metal compact provides a solid skeleton into which liquid metal is introduced during a heating step. The liquid metal is introduced into the metal compact from an external reservoir, such as a mass, e.g., a slug, of the infiltrant metal, that is in contact with a surface of the metal skeleton and is melted during the heating step. The liquid metal is drawn into the open pore structure of the metal skeleton and fills the pores of the skeleton by capillary action. The liquid metal can pass down through the metal skeleton, or can wick up through the metal skeleton.
Examples of infiltrated skeletons include Fe (iron)-Cu (copper) (skeleton-infiltrant), TiC (titanium carbide)-Ni (nickel), Co (cobalt)-Cu, W (tungsten)-Ag (silver), W--Cu, WC (tungsten carbide)-Ag, and Mo (molybdenum)-Ag.
Traditionally, persons skilled in the art considered that infiltration required an open and interconnected pore structure, and thus, the metal skeleton must have at least 10% porosity. See, R. M. German, "Liquid Phase Sintering," Plenum Press, New York, N.Y., pages 160-163 (1985). In addition, the liquid metal infiltrant should have a low viscosity and sufficiently wet the metal skeleton, and no intermediate compounds should form between the metal skeleton and liquid infiltrant because such compounds could block the infiltration path. Ideally, solubility between the metal skeleton and liquid infiltrant is low. To prevent swelling and surface erosion caused by the liquid infiltrant dissolving the metal skeleton as it enters the skeleton, it is common to use a saturated liquid composition as an infiltrant, i.e., an alloy as opposed to a pure metal.
Swelling of the skeleton during infiltration is a common disadvantage, with the magnitude of the swelling related to the metal skeleton porosity, the carbon content of the metal, and the extent of a reaction between the skeleton and infiltrant. Swelling originates from a skeleton infiltrant interaction due to the penetrating liquid infiltrant. Swelling results in a dimensional change of the metal component. Accordingly, such dimensional changes must be factored into the design of the metal component, or secondary process steps are required to machine the metal component back to its correct dimensions. However, the secondary process steps remove the infiltrant from the surface of the skeleton, thereby lowering corrosion resistance. Accordingly, it is important to minimize or eliminate swelling during the infiltration process. The present invention limits swelling to a 2% or less, and typically about a 1% or less, volume increase of the skeleton during infiltration.
The above disadvantages are encountered when infiltrating both magnetic and nonmagnetic metal components manufactured by powder metallurgy. However, magnetic metal components have demonstrated a very serious additional disadvantage when subjected to an infiltration process. In particular, the publication "Effects of Post Processing on DC Magnetic Properties," Leonid I. Frayman, MPIF Magnetic Seminar, May 6-7, 1997, Indianapolis, Ind., (Frayman publication) discloses that infiltrating 410L stainless steel with copper essentially destroyed the magnetic properties of the magnetic metal component. The Frayman publication is discussed in detail hereafter.
J. Ciekot et al., "Powder Metallurgy Materials Featuring Specific Physical Properties," Paper No. 12, Societe Francaise de Metallurgie et de Materiaux, Paris, France, Apr. 6-8, 1992, pages 12-1 to 12-6, (1992), discloses a porous metal skeleton (35-45% porosity) infiltrated with copper. The resulting infiltrated metal component exhibits excellent electric properties, and although the magnetic properties are not adversely affected, the magnetic properties are too low for practical applications. Ciekot et al. disclose that an open or high porosity is necessary to provide suitable electric properties, and that such a porosity is achieved by using a spherical iron powder to manufacture the skeleton.